The present invention relates to a mass spectrometer and a method of mass spectrometry. The preferred embodiment relates to a compact or miniature mass spectrometer in conjunction with an Atmospheric Pressure Ionisation (“API”) ion source.
Conventional mass analysers are normally unable to operate at or near atmospheric pressure and so are located within a vacuum chamber that is evacuated to a low pressure. Most commercial mass analysers operate at a vacuum level of 1×10−4 mbar or lower.
Mass spectrometers with Atmospheric Pressure Ionisation (“API”) ion sources utilise a sampling orifice or capillary in or near the ion source to allow the ions that are created at atmospheric pressure (“AP”) to be admitted into the vacuum chamber containing the mass analyser.
There has been significant development to identify the most efficient method of transferring ions from atmospheric pressure to a vacuum chamber containing the mass analyser. A single orifice between the ion source at atmospheric pressure and the mass analyser is the most direct method but is generally impractical since either the atmospheric pressure orifice needs to be made so small that the number of ions transmitted into the vacuum chamber will be very low (thereby severely restricting the sensitivity of the instrument) or alternatively the mass spectrometer requires an impractically large vacuum pump.
In view of these problems it is common to use one or more stages of differential pumping whereby the pressure is reduced in stages through consecutive vacuum regions each with a small orifice into the adjacent chamber.
It is known to use a rotary vacuum pump to pump a first differential pumping region and one or more turbomolecular vacuum pumps to pump subsequent vacuum regions. Turbomolecular vacuum pumps are unable to exhaust to atmosphere and hence a vacuum pump is required as a backing vacuum pump to the turbomolecular vacuum pump. It is known to use a single rotary vacuum pump to provide pumping for a first stage of differential pumping and also to act as a backing vacuum pump for a turbomolecular vacuum pump.
The sensitivity of a mass spectrometer (which is a key performance characteristic) is closely related to the pumping speeds of the vacuum pumps which are utilised and the gas throughput that the vacuum pumps are able to displace. The pumping speed is the volume flow rate of a vacuum pump and so at higher pumping speed a vacuum pump will be able to displace more gas. Simplistically, vacuum pumps with larger pumping speeds allow mass spectrometers with larger orifices to be constructed (whilst maintaining a similar pressure in a given region) which allow more ions to pass through the orifice thereby increasing the sensitivity of the instrument.
State of the art mass spectrometers have an entrance orifice or capillary(s) that allows a gas throughput from an API ion source into a first differential pumping region of approximately 1000 to 6000 sccm (standard cubic centimeters per minute).
FIG. 1 shows the effect of varying the diameter of an atmospheric pressure sampling orifice upon ion transmission (and hence sensitivity) in relation to a single quadrupole mass spectrometer. In order to generate the data shown in FIG. 1 a valve was used to throttle the pumping in a first differential pumping region in order to keep the pressure in this region the same for each measurement. As can be seen from FIG. 1, a reduction in diameter of the atmospheric pressure sampling orifice from 0.5 mm to 0.15 mm resulted in a reduction in ion transmission to approx. 50%.
FIG. 2 shows the results of a corresponding experiment wherein the diameter of an orifice between first and second stages of differential pumping of a mass spectrometer was varied. As the orifice was reduced from 0.97 mm to 0.6 mm the ion transmission was reduced by >50%.
Conventional single quadrupole mass spectrometers which utilise an Electrospray (“ESI”) ion source use a rotary vacuum pump having a pumping speed in the range 30-65 m3/hr. A turbomolecular vacuum pump with a pumping speed of approximately 300 L/s is commonly used to pump the analyser chamber. The rotary pump also acts as a backing pump to the turbomolecular pump. It will be appreciated that a state of the art single quadrupole mass analyser the mass spectrometer utilises large heavy vacuum pumps. For example, a Leybold SV40 rotary vacuum pump measures 500 mm×300 mm×300 mm and weighs 43 kg and a Pfeiffer splitflow turbomolecular vacuum pump measures 400 mm×165 mm×150 mm and weighs 14 kg.
A compact or miniature mass spectrometer is known and will be discussed in further detail below.
The manufacture of a compact or miniature mass spectrometer advantageously enables physically smaller and lighter vacuum pumps to be utilised. Consequently, these vacuum pumps have lower pumping speeds and therefore in order to maintain the same level of vacuum within the regions of the mass spectrometer as a full size mass spectrometer, smaller orifices must be used. However, replacing a conventional sized orifice with a smaller orifice is problematic since the smaller orifice will have a detrimental effect upon the sensitivity of the instrument. Reducing the sensitivity of the instrument will limit the usefulness of the miniature mass spectrometer and make it less commercially viable.
A known miniature mass spectrometer is disclosed in FIG. 9 of US 2012/0138790 (Microsaic) and Rapid Commun. Mass Spectrom. 2011, 25, 3281-3288. The miniature mass spectrometer as shown in FIG. 9 of US 2012/0138790 comprises a three stage vacuum system. The first vacuum chamber comprises a vacuum interface. The vacuum interface is maintained at a pressure of >67 mbar (>50 Torr) which will be understood by those skilled in the art to be relatively very high. A small first diaphragm vacuum pump is used to pump the vacuum interface.
The second vacuum chamber contains a short RF ion guide which is operated at a pressure-path length in the range 0.01-0.02 Torr·cm and is vacuum pumped by a first turbomolecular vacuum pump which is backed by a second diaphragm vacuum pump. The second separate diaphragm vacuum pump is required due to the relative high pressure (>67 mbar) of the first vacuum chamber. The high pressure in the first vacuum chamber effectively prevents the same diaphragm vacuum pump from being used to back both the first turbomolecular vacuum pump and also to pump the first vacuum chamber due to the fact that turbomolecular vacuum pumps are generally only able to operate with backing pressures of <20 mbar.
The known miniature mass spectrometer therefore requires two diaphragm vacuum pumps in addition to two turbomolecular vacuum pumps or a split-flow turbomolecular vacuum pump.
FIG. 1 of US 2011/02040849 (Wright) discloses a miniature mass spectrometer having a second vacuum chamber which is maintained at a pressure of 10−4 to 10−2 Torr (1.3×10−4 mbar to 1.3×10−2 mbar) and having an ion guide disposed within the second vacuum chamber. The length of the ion guide is not stated.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.